4 Performed for ESD like pulses coupling into a PC motherboard.Immunity scanning results, identifying sensitive traces and comparing two “identical” ICs.Performed for ESD like pulses coupling into a PC motherboard.Color indicates sensitivity.March 2007

8 Eye patternA very effective method of measuring time distortion thru a data transmission system is based on the eye pattern, displayed on an oscilloscope.The eye pattern is simply the superposition - over one unit interval – of all the Zero–to–One and One–to-Zero transitions, each preceded and followed by various combinations of One and Zero, and also constant One and Zero levels.The data sequence can be generated by a pseudo-random sequence generator (PRSG), which is a digital shift register with feedback connected to produce a max length sequence.March 2007

13 Bit error curve as a function of sampling momentEye diagramTimeBERTimeBERBit error curve as a function ofsampling momentHow long would it take if we like to get down to 10e-12 Bit Error Rate?March 2007

14 What Is Jitter?Jitter: The deviation of the significant instances of a signal from their ideal location in time.Or simply, Jitter is how early or late a signal transition is with reference to when it should transition.The significant instances are the transition (crossover) points in a digital signal.Jitter is closely related to phase modulation.P is a wave shape function, e.g., sin or square wave.March 2007

15 What Is Jitter? Phase noise can be analyzed from many points of view:It can be understood as- phase variation (phase noise)- timing variation (time shift of edges)[seconds] mostly used, intuitivesystem description, independentof data rate.- fractions of the period (unit less)[%] Good for directly seeing how manypercent of the eye is openMarch 2007

16 Why Jitter Is Important?Jitter causes transmission errorsor in another wording: Jitter limits the transmission speed.Satisfy jitter budget  BER (bit error rate) targetIdentify jitter components  diminish/decrease deleterious effects on circuit performance from jitterJitter limits the ability of A/D converters!Note:Bit errors can also be caused by voltage noise: If the momentary noise voltage exceeds the noise margin, a wrong value can be sampled even if the sampling takes place at the correct moment in time.March 2007

18 Jitter applications Signal jitter: Timing of asignal (topic of this presentation)Jitter transfer: How strong, as a function of the jitter frequency, ajitter at an input is transmitted toan output in e.g., by a clockrecovery circuitJitter tolerance: How much jitter, as a function of the jitter frequency, can be tolerated by a systemMarch 2007

19 Signal jitter: Why Jitter Is Used?Jitter is not a main system property, but Bit Error Rate (BER) is.Why not measuring BER directly?BER measurement might take hours or days.BER gives little information about the mechanism that cause errors, but jitter does.Is this error caused by jitter?March 2007

20 1101001 1101101 Signal Ideal clock Degraded Noisy clockBit errors can also be caused by voltage noise: If the momentary noise voltage exceeds the noise margin, a wrong value can be sampled even if the sampling takes place at the correct moment in time.March 2007

23 Types of JitterJitter can be random or deterministic. In most cases, both types occur.PJ: Periodic Jitter (deterministic).Is a periodic variation in the phase. Causes: External coupling into the circuit, power supply noise, PLL comparator frequency feed-throughRJ: Random JitterRandom changes in the phase. It is often assumed to be of Gaussian distribution. Causes: Thermal Noise, Shot NoiseMarch 2007Let’s look at some examples

24 Types of JitterJitter can be random or deterministic. In most cases, both types occur.DCD: Duty Cycle Distortion (deterministic).Is the difference in the mean pulse width between positive and negative pulses in a clock. Causes: Amplitude offset, turn-on delay, saturation.ISI: Inter-Symbol Interference (deterministic)Previous signals have not rang down, before new data arrives. Causes: Impulse response is longer than a data bit.March 2007

53 If always the same edge of a bit pattern is observed, then all DDJ is removed. Only non-correlated jitter, RJ and non correlated PJ remains.If averaging is performed over the pattern (not the bits), then all non-correlated jitter: RJ and PJ is removed.March 2007

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